Mercury is the closest planet to the Sun, and the second-smallest planet in the Solar System. Mercury ranges from -0.4 to 5.5 in apparent magnitude, and its greatest angular separation from the Sun (greatest elongation) is only 28.3°, meaning it is only seen in twilight. The planet remains comparatively little-known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40–45% of the planet's surface.